Magnetorheological materials are typically comprised of magnetizable particles in suspension in a fluid carrier. A magnetorheological material characteristically exhibits rapid and reversible changes in rheological properties which can be controlled by the application of a magnetic field. The shear stress and viscosity of such a material are related to whether the material is in the presence of a magnetic field, termed the on-state, or in the absence of a magnetic field, termed the off-state. In the on-state, the magnetizable particles align with the magnetic field and substantially increase the shear yield stress and viscosity of the material over its off-state value. Substantial changes in fluid properties via the application of magnetic fields make possible the use of magnetorheological fluids in many industrial applications.
Commonly, state-of-the-art magnetorheological (MR) fluids are multiphase materials consisting of magnetizable particles suspended in a liquid carrier fluid. In the off-state MR fluids exhibit properties typical of a dense suspension. In addition to the magnetizable particles, the carrier fluid serves as a continuous non-magnetic material. Some of the carrier fluids typically used is silicone or hydrocarbon oils. An additional component that is often present in MR fluids is a stabilizer, which serves to keep the particles suspended in the fluid.
Magnetorheological fluids are useful for wellbore applications but present some drawbacks for wellbore applications; several are briefly described in the following section. The use of magnetorheological fluids in long and vertical fluid columns (e.g. within a conduit) such as those found in wellbores can cause problems because the fluid density is usually greater than that of the well fluids; thus the magnetorheological fluid may sink before being actuated at a predetermined depth. The introduction of field-responsive fluids in a long column can cause significant differential pressure at wellbore depths because of their high density, thus making the deployment of such fluid over great lengths difficult. Another drawback, presented by such fluids arises from the magnetic nature of the suspended particles. Prior to deploying into a well, oilfield tubulars (e.g. pipes, coupling stocks, etc) are commonly magnetic particle inspected (MPI) to ascertain the absence of surface and shallow subsurface defects that could jeopardize structural integrity. This technique of inspection may lead to remnant magnetization thus attracting magnetic particles creating an issue for delivery of magnetic particles downhole. Finally, the fluid cost may also be prohibitive for economic operations in high fluid volume applications.